is your... ?friend?
(If being straitjacket-crazy is kinda a win, then surviving is a win maybe? @3@\ )
More seriously though, for all "central figures" in the Mythos, even witnessing them is Game Over. The best you can do is solving situations their worshippers caused.
That's like trimming the nails of a Dragon just a little bit and saying you've defeated it. As they don't actually care whether we live or not or if we do anything at all, at Marco scale any "win" on our side is purely our own assumption.
(...Unless we're playing Cthulhu d20. That
game was partially famous for having killable Cthulhu cause, ya'know, its 3E. v@w@v )